85070 Idle Air Control (IAC)

							        Idle air control (IAC)




The idle air control (IAC) system tasks
                   To maintain a stable idling speed by directing
                   the correct amount of air past the throttle.
                   To adapt the idling speed to different
                   operating conditions.
 How is the idling speed controlled?
                   The idle air control (IAC) valve (22) has power
                   supply (+) via the system relay.
                   The control module regulates the engine
                   speed (RPM) by grounding the idle air control
                   (IAC) valve. The longer the ground pulse the
                   more the valve opens (= greater airflow) and
                   the greater the engine speed (RPM).
                   Usually the same control module is used to
                   control both the quantity of fuel and the idling
                   speed. Most of the sensors are common to
                   both functions.
                   When is the idling speed correct?
                   Signals from a number of sensors are required
                   in order for the control module to be able to
                   calculate this. Which are the most important
                   sensors for the control of idling speed?




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Basic control of idling speed
              Two sensors are required for this. The throttle
              position (TP) sensor (6). This provides a signal
              indicating closed throttle position (CTP). The
              engine speed (RPM) sensor (2). This indicates
              that the engine speed (RPM) is low; in other
              words idling speed.
              (Closed throttle + high engine speed (RPM) =
              engine braking)
              The control module reads its own memory to
              check what the idling speed should be in the
              current operating condition.
              The control module varies the length of the
              ground pulse to the idle air control (IAC) valve
              (22). This is so that the idling speed is correct
              and kept constant.
              As soon as the throttle is opened slightly the
              idle air control (IAC) valve opens a little extra.
              This to avoid stalling when parking for
              example when the power steering increases
              the load on the engine.
              During engine braking the valve is slowly
              closed to the normal idling speed. This means
              that the negative pressure in the intake
              manifold is limited. In turn this provides
              cleaner exhaust gases.




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Compensating idling speed
            In certain circumstances the idle air control
            (IAC) valve must open a little extra. This is to
            obtain the correct engine speed (RPM).
            Sometimes it must be able to react more
            quickly than normal.
            Cold engine
            When the engine is cold there is greater
            internal friction in the engine. Therefore an
            increased amount of fuel / air mixture is
            required. Some engine variants also require
            higher idling speed.
            The engine coolant temperature (ECT) sensor
            (5) measures the temperature.
            The control module reads its own memory to
            check how much more the idle air control
            (IAC) valve needs to be open for the idling
            speed to be correct despite the higher friction.
            Alternatively, on certain engine variants, the
            idling speed is raised slightly.
            Variation in load at idling speed
            When the load varies considerably at idling
            speed there is a risk of the engine speed
            (RPM) oscillating too much. In such
            circumstance increased preparation and
            quicker control are required.
            On some systems the quantity of fuel is also
            increased temporarily when the load is




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increased This helps the idle air control (IAC)
valve to keep the idling speed as stable as
possible.
When a gear is selected on cars with
automatic transmission a signal is transmitted
by the gear shift selector or the automatic
transmission control module (TCM) (11).
When the air conditioning is selected a signal
is transmitted by the A/C control (14). When
the A/C compressor is engaged and
disengaged signals are transmitted by the
pressure switch (Pressostat) (15).
On some engine variants the idling speed is
increased slightly when the A/C is selected.
This is primarily to increase the power of the
air conditioning system.
This was what was required for idle air trim.




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